This invention relates to a fluid control valve, primarily for use in a vehicle braking system, and operable, in use, to permit the application to a brake of a relatively large volume of fluid for the take-up of brake clearances and/or brake cooling, as well as to permit actuation of the brake by the supply thereto of high pressure fluid. The invention also embraces a braking system incorporating such a valve.
In some conventional braking systems, particularly for commercial or agricultural use, large slave cylinders are employed having a very significant fluid capacity, requiring, in turn, the use of correspondingly large master cylinders. Such arrangements can be disadvantageous, in terms both of the considerable space required to house them on a vehicle and also because operation of the master cylinders can require large pedal efforts or travel.
An object of the invention is to alleviate the aforesaid problems by providing a fluid control valve for use in a vehicle braking system which controls the flow of low pressure fluid to a brake from a fluid source independent of the master cylinder. A further object is to provide a braking system incorporating such a valve.
According to a first aspect of the invention, a fluid flow valve comprises a body having first, second and third fluid inlets, a working fluid outlet, a first pressure element movable, under the influence of pilot pressure fluid applied to the first inlet, from a first position in which it causes closure of the second inlet, to a second position in which it causes opening of the second inlet, and a second pressure element movable, in response to pressure at the fluid outlet, from a first position in which it causes closure of the third inlet to a second position in which it causes opening of the third inlet, said elements operating in conjunction to permit an initial high volume fluid flow from the second inlet through the outlet and subsequently, with the second element causing interruption of a path between the second inlet and outlet, a high pressure fluid flow from the third inlet through the outlet.
In one convenient arrangement, a further outlet of the body for connection to an external low pressure region is connected within the body to the working outlet when the first pressure element is in its first position and prevented from communicating with the working outlet by the first pressure element when in its second position.
Preferably, the pressure elements are pistons slidable within the body and, conveniently, telescoped one within the other.
From another aspect of the invention, a vehicle braking system comprises a fluid control valve in accordance with the aforesaid first aspect of the invention, a source of pilot pressure fluid connected to the first inlet, a fluid source capable of providing a relatively high volume supply of fluid connected to the second inlet, a high pressure fluid source connected to the third inlet and a brake connected to the working fluid outlet, the arrangement being such that pilot pressure applied via the first inlet to the first pressure element causes movement of the latter to open the second inlet for admission of a high volume fluid flow through the working fluid outlet, the second fluid pressure element being exposed to and moved by the pressure of the high volume fluid acting thereon to close the second inlet and open the third inlet through which high pressure fluid is then admitted to the brake via the working fluid outlet for brake actuation.
Conveniently, a hydrostatic master cylinder may be the source of both the pilot pressure and high pressure fluid, the high volume fluid being typically provided from a hydrodynamic fluid source.
Alternatively, a hydrodynamic fluid supply may be used, conveniently via a power valve, to provide both the high volume fluid and high pressure fluid. It would also be possible to derive the pilot pressure from the hydrodynamic supply via the power valve.